The use of a fluorinated ion exchange membrane having carboxylate and/or sulfonate ion exchange groups, as a membrane for separating the anode and cathode compartments of a fuel cell or an electrolytic cell, especially a chloralkali electrolytic cell, is well known. In an electrolytic cell, it is desired that the ion exchange membrane exhibit low cell voltage and high current efficiency, thereby enabling the electrolytic cell to be stably operated with low electric power consumption. In a fuel cell, it is desired that the ion exchange membrane exhibit high ionic conductivity, thereby enabling the fuel cell to be stably operated with high electric power output.
The fluorinated ion exchange membrane having carboxylate and/or sulfonate ion exchange groups is conventionally manufactured by first molding a fluorinated polymer having ion exchange precursor groups and having thermoplastic properties into a film, and then hydrolyzing the ion exchange precursor groups to thereby form carboxylate and/or sulfonate ion exchange groups. The customary method for hydrolyzing the ion exchange precursor groups comprises contacting the precursor groups with an aqueous solution of an alkali metal hydroxide or with a mixture of an aqueous solution of an alkali metal hydroxide and an organic solvent selected from an alcohol such as methanol, ethanol or propanol or a water soluble organic solvent such as dimethyl sulfoxide. The alcohol or the water soluble organic solvent is added in order to increase the hydrolysis rate of the ion exchange precursor groups.
It is known that an ion exchange membrane is likely to swell, thereby forming wrinkles on the surface of the membrane, when it comes into contact with a cell electrolyte. This wrinkle formation is likely to be accompanied by problems, such as increase in voltage of an electrolytic cell attributed to the retention of evolved gas and/or electrolyte by the wrinkles, pinhole formation, and membrane tearing attributed to the rubbing of the wrinkled membrane against an electrode. To cope with these problems, proposals have been made in which an ion exchange membrane is pre-swollen prior to installation in the cell by immersing the membrane in a specific organic solvent or an aqueous solution of an organic solvent. For example, U.S. Pat. No. 4,595,476 discloses a process for pre-swelling an ion exchange membrane with an aqueous solution containing an organic solvent such as diethylene glycol and triethylene glycol. Further, U.S. Pat. No. 4,376,030 discloses a process for pre-swelling an ion exchange membrane in which the membrane is pre-swollen at a temperature of from 20.degree. to 80.degree. C. using an aqueous solution containing an amine selected from primary, secondary and tertiary amines, most preferably triethanolamine, in an amount of from 2 to 60% by weight.
However, in these prior art hydrolysis processes, two separate steps, i.e. a hydrolysis step and a pre-swelling step, are inevitably involved. Consequently, all of these processes have a drawback in that a complicated procedure is necessary.
U.S. Pat. No. 4,904,701 discloses a hydrolysis process in which the membrane is hydrolyzed with an aqueous solution of at least one basic organic compound for a period of time sufficient to hydrolyze the precursor ion exchange groups. The basic organic compounds include basic nitrogen compounds such as an amine or an imine, preferably triethanolamine or diethanolamine. However, the basic organic compound may not completely hydrolyze the precursor ion exchange groups or the hydrolysis process may take a long time. In addition, the basic organic compound may adversely interact with the ion exchange change groups, thereby decreasing the stability and efficiency of the membrane. In addition, amines such as triethanolamine may be flammable or explosion hazards.
U.S. Pat. No. 5,066,682 discloses a hydrolysis process in which the membrane is hydrolyzed with an alkali aqueous solution containing dimethyl sulfoxide, methanol, ethanol or propanol. However, methanol, ethanol and propanol are difficult to handle due to flammability and explosion hazards. In addition, high concentrations, up to 30% by weight, may be necessary for the additive to facilitate hydrolysis. Dimethyl sulfoxide may be toxic and also causes noxious odors. Dimethyl sulfoxide may also contribute to poisoning of the electrode catalyst in an electrochemical cell, particularly a cell having a unified membrane and electrode structure. Moreover, dimethyl sulfoxide may be difficult to dispose of, because it may cause noxious odors during incineration or in an anaerobic treatment facility.
Therefore, a hydrolysis process for ion exchange membranes is needed in which the membrane may be hydrolyzed in a quick, simple manner, the membrane is dimensionally stable and is pre-swollen in order to avoid wrinkles and pinholes, the hydrolysis agent does not adversely interact with the membrane's ion exchange groups, the hydrolysis agent is not highly flammable, does not poison the electrode catalyst and may be readily disposed of.